1. Field of the Invention
The present invention relates to a hot-wire TIG arc welding apparatus and, more particularly, to an inexpensive and widely usable hot-wire TIG arc welding apparatus.
2. Description of the Prior Art
As well known to those skilled in the art, in a hot-wire TIG arc welding method, a tungsten electrode and a base metal disposed in an inert shield gas such as argon gas are connected to an arc generating electric power source having a D.C. drooping characteristic such that the tungsten electrode and the base metal constitute a negative electrode and a positive electrode, respectively, so that an arc is formed in the argon shield gas to melt the base metal. In ordinary TIG arc welding, a wire of the welding material is simply fed to the melt of the base metal continuously. On the other hand, in a specific form of TIG arc welding generally known as "hot-wire TIG arc welding", a D.C. or an A.C. power is supplied to the wire which is being fed to the arc generating area from the wire feeding device such that Joule heat is generated in the wire thereby increasing the rate of melting of the wire.
In the TIG arc welding there has been caused a problem of magnetic blow phenomenon of arc which phenomenon is caused due to wire current. That is, when supplying wire heating current, the arc largely swings back and forth due to the electromagnetic interaction between the arc and wire heating current, with the result that a molten pool of a weld metal becomes unstable so that suitable welding can not be effected. Even in the case where A.C. is used as wire heating current, the arc can not be set to the center of the molten pool, so that suitable welding can not be effected. In order to effect suitable welding without substantial influence of the magnetic blow phenomenon, it is necessary to reduce the value of wire heating current to become one-half or less of arc current value, so that the rate of wire-melting is limited to the degree of 20 g/min. On the other hand, in order to minimize the magnetic blow phenomenon while increasing the rate of wire-melting, there is a hot-wire switching TIG welding as shown, for example, in U.S. Pat. No. 3,627,974 Specification dated Dec. 14, 1971. The inventors of the present invention have already invented a modified method of the hot-wire switching TIG welding in which the arc current is made to become pulsed current having both a peak value period and a base value period, power supply to the wire being not effected at the time of the peak current period, the power supply to the wire being effected only at the time of the base current period. According to the modified method, it becomes possible to obtain a wire melting rate of not less than 100 g/min without substantial influence of the magnetic blow phenomenon.
FIG. 1 shows an example of the power supply system for the hot wire switching TIG arc welding system which needs no exclusive power source. Namely, in the power supply system shown in FIG. 1, a switching unit employing a gate turn off (GTO) thyristor is combined with a commercially available TIG arc welding power supply system such that the hot wire switching welding may be effected. More specifically, a welding power source 3 having a D.C. drooping characteristic is connected between a torch 4 and a base metal 2 such as to form an arc 5 therebetween, and a first GTO thyristor 16 is connected to the wire 6 in parallel to the arc, so that the wire is heated periodically by consecutive turning on and off of the first GTO thyristor 16. Namely, when the first GTO thyristor 16 is turned on, the electric current supplied from the power source 3 as the arc current is short-circuited through the wire 6 thereby heating the wire 6. The welding cannot be restarted once the arc 5 is extinguished. Thus, in order to prevent extinction of the arc 5 during the heating of the wire 6, a base power source 18 is connected between the torch 4 and the base metal 5. The control for the optimum heating of the wire 6 is conducted by suitably short-circuiting the heating current through a second GTO thyristor 17 which is turned on and off in a controlled manner. The power supply system shown in FIG. 1 employs various other constituents such as a tungsten electrode 1, contact tip 7, wire feeding reel 11, gate control circuit 19, and a switching unit 20.
The power supply system shown in FIG. 1, however, is rather expensive due to the use of expensive current switching device of large capacity such as the GTO thyristors, although it can make use of a commercially available power source as the arc power source.
Under this circumstance, a hot wire switching TIG arc welding method has been proposed in which the wire heating power supply system is constituted by inexpensive semiconductor switches, e.g., triac, composed of usual thyristors, designed to effect the control of the phase of electric power supplied to the wire. FIG. 2 shows the principle of this type of wire heating power supply system. Namely, this power supply system incorporates an external controller 22 which produces a signal in the form of pulses which controls the TIG arc power source 21 and the wire heating power source 23 in such a manner that the wire is not supplied with the power during a period in which the current of the arc 5 takes the peak level (referred to as "peak period", hereinunder) but is supplied with the power during another period in which the current of the arc 5 takes the base level (referred to as "base period", hereinunder).
This type of power supply system, however, suffers from the following disadvantage. Namely, since this power supply system makes use of various types of commercially available TIG arc power supplies as the arc power supply 21, it is necessary to effect a suitable modification of the control circuit in the TIG arc welder used. The manner of modification of the control circuits varies one-by-one depending on the type of the welder and a study from a technical point of view is necessary for the determination of the manner of modification of the control circuit for the individual TIG arc welder.